Microphone arrays capture the signals from multiple sensors and process those signals in order to improve the signal-to-noise ratio. In conventional beamforming, the general approach is to combine the signals from all sensors (channels). One typical use of beamforming is to provide the combined signals to a speech recognizer for use in speech recognition.
In practice, however this approach can actually degrade the overall performance, and indeed, sometimes performs worse than even a single microphone. In part this is because of individual hardware differences between the microphones, which can result in different microphones picking up different kinds and different amounts of noise. Another factor is that the noise sources may change dynamically. Still further, different microphones deteriorate differently, again leading to degraded performance.